Transdermal administration of drugs has been normally achieved by using formulations for transdermal administration in the form of solutions or ointments to be applied or plastered to the skin surface. The skin, in the case of human, normally comprises a number of tissues: the stratum corneum (horny layer) having a laminar structure in a thickness of 10 to 30 μm, the epidermal tissue layer in a thickness of about 70 μm, and the dermal tissue layer in a thickness of about 2 mm.
The stratum corneum, which is a laminar structure constituting the outermost layer of the skin, functions as a barrier to prevent various drugs from penetrating the skin. Generally, about 50 to 90% of the barrier action of the skin is attributed to the stratum corneum. The epidermal layer does not contribute to the barrier action as great as the stratum corneum; however it accounts for the remaining about 10% or more of the skin barrier action. Meanwhile, the dermal layer has an extensive capillary network in the vicinity of the junction between the dermal layer and the epidermal layer. Once a drug reaches the depth of the dermis, the drug quickly diffuses into deeper tissues (hair follicles, muscles and the like) through the capillary network. Then, the drug is systematically diffused through the blood circulation from the capillaries.
Currently, there have been developed various formulations for transdermal administration in the form of solutions or ointments to be applied or plastered to the skin surface. However, due to the barrier action of the stratum corneum described above, absorption of drug-effective components is insufficient. For example, it is said that, even formulations for transdermal administration of indomethacin, which are considered to exhibit high transdermal absorption rate, enable only about 5% of the total amount of indomethacin to be absorbed transdermally.
Accordingly, as one method to increase the skin permeability of a drug, as shown in Patent Document 1, it has been tried to topically destroy the stratum corneum using small needles (microneedle or microsyringe) to forcibly deliver the drug into the dermal layer.
The small needles used for this purpose have preferably 30 μm or more in length in order that they reach the dermal layer, and the small needles are considered to require a base for supporting the needles. Since the small needles do not reach the dermal layer where nerve terminals are located, they do not cause pain. Therefore, there is an advantage of administering a drug without causing fear to children, etc.
While a wide variety of methods for producing the microneedle have been studied, there also have been studied devices (i.e., assistive tool) for puncturing skin with the microneedle effectively. These devices employ a method of using impact force of a spring or the like to puncture skin with the microneedle (Patent Documents 2 and 3).
These puncturing methods of the microneedle using impact force, however, may cause easy breakage of the small needles of the microneedle in the step of puncturing the skin and also cause excessive damage to the skin surface. Therefore, there is a demand for devices that cause less impact and can be punctured softly. Accordingly, an array device in the form of a syringe has been reported, in which a piston is inserted into a tube and pressed by hand to puncture the skin slowly (Patent Document 4).    Patent Document 1: JP-A-2006-149818    Patent Document 2: JP-A-2008-520369    Patent Document 3: JP-A-2008-543527    Patent Document 4: WO 2008/069566